Lithography system and method thereof

ABSTRACT

A method includes generating a plasma that emits a first EUV radiation in a vessel at a first gas exhaust rate of the vessel; directing the first EUV radiation to a first substrate using a collector in the vessel; halting the generating of the first EUV radiation; and ejecting a gas past the collector at a second gas exhaust rate of the vessel, in which the second gas exhaust rate is greater than the first gas exhaust rate after the halting.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofIC processing and manufacturing. For these advances to be realized,similar developments in IC processing and manufacturing are needed. Forexample, the need to perform higher resolution lithography processesgrows. One lithography technique is extreme ultraviolet lithography(EUVL). Other techniques include X-Ray lithography, ion beam projectionlithography, electron beam projection lithography, and multiple electronbeam maskless lithography.

The EUVL employs scanners using light in the extreme ultraviolet (EUV)region, having a wavelength of about 1-100 nm. Some EUV scanners provide4× reduction projection printing, similar to some optical scanners,except that the EUV scanners use reflective rather than refractiveoptics, i.e., mirrors instead of lenses. EUV scanners provide desiredpatterns on wafers by transferring mask patterns defined by an absorberlayer. Currently, binary intensity masks (BIM) accompanied by on-axisillumination (ONI) are employed in EUVL. In order to achieve adequateaerial image contrast for future nodes, e.g., nodes with the minimumpitch of 32 nm and 22 nm, etc., several techniques, e.g., the attenuatedphase-shifting mask (AttPSM) and the alternating phase-shifting mask(AltPSM), have been developed to obtain resolution enhancement for EUVL.But each technique has its limitation needed to be overcome. Forexample, an absorption layer however may not fully absorb the incidentlight and a portion of the incident light is reflected from theabsorption layer. Also the thickness of the absorption layer causes theshadowing effect. All of these often result in reduced aerial imagecontrast, which may lead to poor pattern profiles and poor resolution,particularly as pattern features continue to decrease in size. It isdesired to have improvements in this area.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying Figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a method in accordance with some embodiments of the presentdisclosure.

FIG. 2A is a schematic view of lithography system in accordance withsome embodiments of the present disclosure.

FIG. 2B is a schematic view of lithography system in accordance withsome embodiments of the present disclosure.

FIG. 3 is schematic view of a EUV lithography system including analignment module that is used for a photomask in accordance with someembodiments of the present disclosure.

FIG. 4 is schematic view of a EUV lithography system including analignment module that is used for a substrate in accordance with someembodiments of the present disclosure.

FIG. 5 is schematic view of a EUV lithography system including analignment module with a light source in accordance with some embodimentsof the present disclosure.

FIG. 6 is a graph showing an open ratio of a valve as a function of timefor the exemplary structure as shown in FIGS. 2A-5 with some embodimentsof the present disclosure.

FIG. 7 is a schematic view of a computer including a controller inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the Figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe Figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 illustrates an exemplary method M in accordance with someembodiments. The method M includes a relevant part of the entiremanufacturing process. The method M may be implemented, in whole or inpart, by a system employing extreme ultraviolet (EUV) lithography andother appropriate lithography processes to improve pattern dimensionaccuracy. Additional operations can be provided before, during, andafter the method M, and some operations described can be replaced,eliminated, modified, moved around, or relocated for additionalembodiments of the method. One of ordinary skill in the art mayrecognize other examples of semiconductor fabrication processes that maybenefit from aspects of the present disclosure. The method M is anexample, and is not intended to limit the present disclosure beyond whatis explicitly recited in the claims.

The method M is described below in conjunction with FIGS. 2A-5. FIGS.2A-5 illustrate various stages of the method M according to someembodiments of the present disclosure. The method M begins at block S101where a plasma is generated and emits a first EUV radiation in a vesselat a first gas exhaust rate, in which the plasma generates debris.Referring to FIG. 2A, in some embodiments of block S101, shown there isa EUV lithography system 10. Although the EUV lithography system 10 isillustrated as having a certain configuration of components, it will beappreciated that the disclosed EUV lithography system 10 may includeadditional components (e.g., additional mirrors) or having lesscomponents (e.g., less mirrors).

In FIG. 2A, the EUV lithography system 10 includes a source collectormodule SO that includes a source vessel 110. A droplet generator 120 isconnected to the source vessel 110 and is configured to generate aplurality of fuel droplets 112. In some embodiments, the fuel droplets112 generated by the fuel droplet generator 120 are provided into theEUV source vessel 110. In some embodiments, the fuel droplets 112 mayinclude tin (Sn). In other embodiments, the fuel droplets 112 mayinclude a different metal material. In some embodiments, the sourcevessel 110 can also be referred to as a radiation source, in whichradiation source employs a laser produced plasma (LPP) mechanism togenerate plasma and further generate EUV light from the plasma. In someembodiments, the first gas exhaust rate in the source vessel 110 shownin FIG. 2A is less than about 400 L/s. In some embodiments, a pressurein the source vessel 110 shown in FIG. 2A is in a range from about 1 mmBar to about 3 mm Bar in response to the first gas exhaust rate. Thus, aselected gas number density in the irradiation region 122 of the sourcevessel 110 and/or a selected gas composition, e.g. a selected ratio ofseveral gases, e.g. H₂, HBr, He, etc can be maintained. Therefore,amount of the plasma 114 can be generated for creating effective andefficient EUV light.

In some embodiments, the first gas exhaust rate in the source vessel 110shown in FIG. 2A is less than about 450 L/s to maintain a selected gasnumber density in the irradiation region 122 of the source vessel 110and/or a selected gas composition. In some embodiments, the first gasexhaust rate in the source vessel 110 shown in FIG. 2A is less thanabout 350 Ls to maintain a selected gas number density in theirradiation region 122 of the source vessel 110 and/or a selected gascomposition.

The EUV lithography system 10 may also include a droplet positiondetection system which may include a droplet imager 140 disposed in thesource vessel 110 that captures an image of one or more fuel droplets112. The droplet imager 140 may provide this captured image to a dropletposition detection feedback system (not shown), which can, e.g.,generate a droplet position and trajectory in response an analysisresult of the captured image. The position detection feedback system canthus generate a droplet error in response to the generated dropletposition and trajectory, e.g., based on a droplet-by-droplet basis, oron average. In some embodiments, the droplet imager 140 may include afine droplet steering camera (FDSC), a droplet formation camera (DFC),and/or suitable devices.

The EUV lithography system 10 further includes a primary laser having alaser source 102 configured to produce a laser beam 104. In someembodiments, the laser source 102 may include a multi-stage laser havinga plurality of stages configured to amplify laser light produced by aprior stage. The laser beam 104 passes through a beam transport system106 configured to provide the laser beam to a focusing system 108. Thebeam transport system 106 receives the light beam 104 and steers andmodifies the light beam 104 as needed and outputs the light beam 104 tothe focusing system 108. The focusing system 108 includes one or morelenses 108 a, 108 b and/or mirrors arranged within a beam line andconfigured to focus the laser beam 104. The laser beam 104 is outputfrom the focusing system 108 to the EUV source vessel 110 along a driveaxis toward an irradiation region 122. The f focusing system 108 canalso steer the beam 104 or adjust a position of the beam 104 relative tothe irradiation region 122. The drive axis of the amplified laser beam104 can be considered as the approximate center of the laser beam 104 orthe general direction that the laser beam 104 is traveling because thelaser beam 104 may be irregularly shaped and/or asymmetrical.

The laser beam 104 transmits through a collector mirror 118 locatedwithin the source vessel 110. Then, the primary laser beam 104 generatedby the laser source 102 intersects the fuel droplets 112. In someembodiments, the primary laser beam 104 may be a carbon dioxide (CO₂)laser. In other embodiments, the primary laser beam 104 may includealternative types of lasers. When the primary laser beam 104 strikes thefuel droplets 112, the primary laser beam 104 heats the fuel droplets112 to a critical temperature. At the critical temperature, the fueldroplets 112 shed their electrons and become a plasma 114 including aplurality of ions at the irradiation region 122. In some embodiments,the ions emit EUV radiation 116 (e.g., having a wavelength ofapproximately 13.3 nm to about 13.7 nm).

By-products of the EUV radiation 116 may include metal dust, targetmaterial vapor and micro-droplets or clusters and can be in severalforms, for example, when tin, e.g., pure tin, or a tin compound, e.g.,SnBr₄, SnH₄, SnBr₂ etc, is used as the source material, the by-productsmay include tin and tin compounds including oxides. Dusts and othercontaminates, e.g., from collector mirror 118 erosion, etc. may also bepresent in the source vessel 110. These by-products may, among otherthings, damage optics and absorb/scatter EUV radiation 116. In someembodiments, target material debris deposits can be present in manyforms. By way of example, particulates, can deposit on the surface ofthe collector mirror 118.

In some embodiments, the collector mirror 118 has a concave curvature.In some embodiments, the collector mirror 118 includes a center aperture119. The center aperture 119 allows the primary laser beam 104 to passthrough to an irradiation region 122. In some embodiments, the collectormirror 118 may include a multi-layer coating having alternating layersof different materials. For example, in some embodiments, the collectormirror 118 may include alternating layers of molybdenum and siliconconfigured to operate as a Bragg reflector. The concave curvature of thecollector mirror 118 focuses the EUV radiation 116 generated by theplasma 114 toward an intermediate focus (IF) unit 130 within an exitaperture of the source vessel 110. The intermediate focus unit 130 islocated between the source vessel 110 and a scanner 200 includingoptical elements configured to direct the EUV radiation 116 to aworkpiece (e.g., a semiconductor substrate). In some embodiments, theintermediate focus unit 130 may include a cone shaped apertureconfigured to provide for separation of pressures between the sourcevessel 110 and the scanner 200. In some embodiments, the intermediatefocus unit 130 may extend into the scanner 200 which including anillumination system IL and a projection system PS. The EUV radiation 116output from the source vessel 110 is provided to an illumination opticsunit IL by way of the intermediate focus unit 130.

Returning to FIG. 1, the method M then proceeds to block S102 where thefirst EUV radiation is directed to a first substrate. With reference toFIG. 2A, in some embodiments of block S102, the EUV radiation 116traverses the illumination system IL, which may include a facetted fieldmirror device 220 and a facetted pupil mirror device 240 arranged toprovide a desired angular distribution of the radiation beam 210, at thepatterning device MA, as well as a desired uniformity of radiationintensity at the patterning device MA. Upon reflection of the radiationbeam 210 at the patterning device MA, held by the support structure MT,a patterned beam 260 is formed and the patterned beam 260 is imaged bythe projection system PS via reflective elements 280, 290 onto asubstrate W1 held by the substrate table WT.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam that is reflected by the mirrormatrix.

More elements than shown may generally be present in illumination opticsunit IL and projection system PS. A grating spectral filter mayoptionally be present, depending upon the type of lithographicapparatus. Further, there may be more mirrors present than those shownin the Figures, for example there may be 1-6 additional reflectiveelements present in the projection system PS than shown in FIG. 2A.

The EUV lithography system 10 may also include an EUV energy monitor 150disposed in the source vessel 110. The EUV energy monitor 150 isdesigned to monitor the EUV intensity or energy generated from thesource vessel 110. For example, the EUV energy monitor 150 includes anEUV sensing element, such as a diode, designed to be sensitive to theEUV light and configured to effectively detect the EUV light. In otherexamples, the EUV energy monitor 150 includes a plurality of diodesconfigured in an array to effectively detect the EUV light formonitoring purpose. In some embodiments, a dose error is calculatedbased on the sensed EUV intensity (or energy). For example, when thesensed EUV intensity (or energy) is below a predetermined thresholdvalue, such situation can be referred to as a dose error. Generally, thedose error is related to the plasma instability, through monitoring theEUV intensity by the EUV energy monitor 150, the dose error can beextracted from the monitored EUV intensity. Therefore, when a dose erroris occurred, it indicates that the plasma 114 is unstable.

In some embodiments, the EUV lithography system further includes adroplet collection element 125 disposed in the source vessel 110 andlocated opposite to the droplet generator 120. The droplet collectionelement 125 is configured to collect fuel droplets 112 that are notvaporized during formation of the EUV radiation 116 and/or fragments offuel droplets 112 generated during formation of the EUV radiation 116.

Returning to FIG. 1, the method M then proceeds to block S103 where agas provided in the vessel at the first gas exhaust rate duringgenerating of the plasma. With reference to FIG. 2B, in some embodimentsof block S103, the source collector module SO may include a regulatedgas source 222 for selectively introducing, either continuously or indiscrete amounts, one or more gas(es) into the source vessel 110, e.g.for ion stopping (e.g. H₂, (protium and/or deuterium isotopes) and/orHe). It is to be appreciated that the gas source 222 may include one ormore flow regulators (not shown). In some embodiments, a purge gassupply mechanism may fix the flow rate of the purge gas from the gassource 222 at a constant level by using the flow rate control valve suchas a throttle valve, and the MFC or the pressure control loop may beomitted.

In some embodiments, the gas source 222 is disposed proximate to thecenter aperture 119 such that gas 271 generated therein are outputthrough the center aperture 119 of the collector mirror 118. In someembodiments, the gas 271 includes hydrogen and/or hydrogen radicals H*.For example, the gas 271 generated central in situ hydrogen radicalsources provide hydrogen radicals proximate to the target materialdebris deposited on the collector mirror 118 and near the centeraperture 119. It should be noted that the gas 271 can be any sort ofhydrogen radical source described herein. In some embodiments, the tinin the target material debris deposits converts to volatile tincompounds such as SnH₄ by the hydrogen radicals H*. In some embodiments,the gas source 222 has a signal source and a hydrogen nozzle injectshydrogen into a magnetic field generated by an induction coil to createa hydrogen plasma that produces the hydrogen radicals H*. In someembodiments, the gas 271 may be also referred to as a carrier gas.

In FIG. 2B, it can be seen that within each closed loop flow path, gasis directed through the center aperture 119 formed in the collectormirror 118 and toward the irradiation region 122. Although, other centerapertures may be provided and used to flow gas through the collectormirror 118. Moreover, other suitable flow paths may be establishedwithin the source vessel 110. A portion of the gas 271 from the centeraperture 119 may flow within source vessel 110 through a vane structure214.

As shown in FIG. 2B, the vane structure 214 may be disposed between thecollector mirror 118 and the intermediate focus unit 130 and may includea plurality of vanes that are arranged to allow light to travel from thecollector mirror 118 to the intermediate focus unit 130. In someembodiments, the vane structure 214 may be formed with internal passagesto flow a heat exchange fluid, e.g. water or liquid gallium, to cooleach vane. The vane structure 214 may function to cool gas flowingthrough thereof and/or to condense target material vapors that mayundesirably absorb EUV radiation, e.g. tin vapor when tin is used as atarget material and/or to provide significant resistance to gas flow,thus, establishing a pressure gradient in the source vessel 110 with arelatively high gas pressure upstream of the vane structure 214, e.g.between the irradiation region 122 and collector mirror 118 to e.g.provide ion stopping and/or etching power, and a relatively low gaspressure downstream of the vane structure 214, e.g. between the vanestructure 214 and the intermediate focus unit 130, to e.g. minimize EUVabsorption.

FIG. 2B further shows that the source collector module SO may includeexternal guideways 204 a and 204 b, a source valve 208, an adjustablepump 224, and a guideway 209 between the source valve 208 and the pump224. The pump, e.g. turbopump or roots booster is used for selectivelyremoving some or all of the gas from the source vessel 110 through thepump 204. Removal of gas via pump 224 from the source vessel 110 may beperformed to remove contaminants, vapor, metal dust, etc. from thesource vessel 110, and/or to provide a pressure gradient in the sourcevessel 110, e.g. to maintain a relatively large pressure between thecollector mirror 118 and irradiation region 122 and a smaller,relatively low pressure between the irradiation region 122 and theintermediate focus unit 130.

In some embodiments, the gas exhaust rate is controlled by controlmechanisms including the use of an exhaust gas control valve such as anelectromagnetic valve, or a gas exhaust means such as a suction pump,and the use of a flap disposed in an exhaust pipe, the degree of openingof said flap being controlled. For example, the gas exhaust rate iscontrolled by the source valve 208 with the pump 224 as shown in FIG.2B. In some embodiments, the size of an opening of the exhaust sectionis consecutively changed so as to control accurately the gas exhaustrate.

In some embodiments, the EUV lithography system 10 permits changing thegas exhaust rate in accordance with different stage of the operationthereof. The size of the opening of the source valve 208 as shown inFIG. 2B is controlled to permit the gas exhaust rate to change. The gasexhaust rate is continuously detected by the exhaust rate detectingmeans such as gas monitors 228 and 229, and the size of the opening iscontrolled to conform with the detected gas exhaust rate.

Referring to FIG. 2B, The EUV lithography system 10 further includes amaster controller 157, a control unit 155, and a converter 158 connectedbetween the control unit 155 and the source valve 208. The mastercontroller 157 is configured to determine whether the amplified laserbeam 104 (e.g., laser pulses) generated from the laser source 102intercepts the fuel droplets 112 generated from the droplet generator120 (shown in FIG. 2A). As shown in FIG. 2B, the master controller 157is determined that the laser pulses properly intercept the droplets inthe right place and time for effective and efficient EUV lightproduction. When the amplified light beam 110 strikes the fuel droplets112, the fuel droplets 112 is converted into a plasma state that has anelement with an emission line in the EUV range. In some embodiments, thepulses of the laser source 102 and the droplet generating rate of thefuel droplet generator 120 are controlled to be synchronized such thatthe fuel droplet generator 120 receive peak powers consistently from thelaser pulses of the laser source 102. In some examples, the dropletgeneration frequency ranges from 20 kHz to 100 kHz. For example, thelaser source 102 includes a laser circuit designed to control thegeneration of the laser pulses. The laser circuit and the fuel dropletgenerator 120 are coupled to synchronize the generation of the laserpulses and the generations of the Tin droplets.

At the same time, the master controller 157 receives an output signalfrom the energy sensors 170 (shown in FIG. 2A) and performs an analysisbased at least in part on this received output to actuate the controlunit 155. In some embodiments, the control unit 155 further actuates thescanner 200 to perform a photolithography process on the substrate W1.The control unit 155 therefore provides a signal to the converter 158 toadjust an open ratio (cross-section ratio of the passages and material,e.g., gas) of the source valve 208. In some embodiments, the open ratioof the source valve 208 is in a range from about 20% to about 30%. Thus,control of the gas source 222 and the pump 224 in response to theadjustment of the source valve 208 may be used to maintain a selectedgas number density in the irradiation region 122 of the source vessel110 and/or pressure gradient and/or to maintain a selected flow ratethrough the source vessel 110 and or to maintain a selected gascomposition, e.g. a selected ratio of several gases, e.g. H₂, HBr, He,etc. Therefore, amount of the plasma 114 can be generated for creatingeffective and efficient EUV light.

In some embodiments, the open ratio of the source valve 208 ismaintained during the photolithography process on the substrate W1 andduring the providing of the gas 271 to the source vessel 110. Further,the gas exhaust rate of the source vessel 110 is maintained, and thusthe pressure in the source vessel is substantially a constant.

FIG. 2B further shows that gas monitors 228 and 229 measuring one ormore gas characteristic including, but not limited to, gas temperature,pressure, composition, e.g. He/H₂ ratio, HBr gas concentration, etc. maybe disposed in the source vessel 110 or placed in fluid communicationtherewith to provide one or more signals indicative thereof to a gasmanagement system controller (not shown), which, in turn, may controlthe pumps, regulators, etc. to maintain a selected gas temperature,pressure and/or composition. Specifically, the gas monitor 228 isdisposed within the vane structure 214 and above the collector mirror118. The gas monitor 229 is disposed between the vane structure 214 andan inner sidewall of the source vessel 110 and adjacent to the guideways204 a and/or 204 b.

In some embodiments, the master controller 157 and/or a memory 156connected to the master controller 157 either one of them may be part ofa computer assembly as described with reference to FIG. 7. It should beunderstood that a master controller 157 as used throughout this text canbe implemented in a computer assembly as shown in FIG. 7. The memory 156may comprise a number of memory components like a hard disk 341, ReadOnly Memory (ROM) 342, Electrically Erasable Programmable Read OnlyMemory (EEPROM) 343, and/or Random Access Memory (RAM) 344. Not allaforementioned memory components need to be present. Furthermore, it isnot essential that aforementioned memory components are physically inclose proximity to the master controller 157 or to each other. In someembodiments, the aforementioned memory components may be located at adistance away.

The master controller 157 may also be connected to some kind of userinterface, for instance a keyboard 345 or a mouse 346 shown in FIG. 7. Atouch screen, track ball, speech converter or other interfaces that areknown to persons skilled in the art may also be used.

The master controller 157 may be connected to a reading unit 347 shownin FIG. 7, which is arranged to read data from and under somecircumstances store data on a data carrier, like a CDROM 349. Also DVD'sor other data carriers known to persons skilled in the art may be used.

The master controller 157 may also be connected to a printer 350 shownin FIG. 7 to print out output data on paper as well as to a display 351,for instance a monitor or LCD (Liquid Crystal Display), of any othertype of display known to a person skilled in the art.

The master controller 157 may be connected to a communications network352 shown in FIG. 7, for instance a public switched telephone network(PSTN), a local area network (LAN), a wide area network (WAN) etc. byway of transmitters/receivers 353 responsible for input/output (I/O).The master controller 157 may be arranged to communicate with othercommunication systems via the communications network 352. In someembodiments of the external computers (not shown), for instance personalcomputers of operators, can log into the master controller 157 via thecommunications network 352.

The master controller 157 may be implemented as an independent system oras a number of processing units that operate in parallel, wherein eachprocessing unit is arranged to execute sub-tasks of a larger program.The processing units may also be divided in one or more main processingunits with several subprocessing units. Some processing units of themaster controller 157 may even be located a distance away of the otherprocessing units and communicate via communications network 352 shown inFIG. 7.

Returning to FIG. 1, the method M then proceeds to block S104 where thegeneration of the first EUV radiation is halted. With reference to FIG.3, in some embodiments of block S104, the master controller 157 isdetermined that the laser pulses generated from the laser source 102 arespaced apart from the fuel droplet generator 120 as shown in FIG. 2A,and thus there is no EUV radiation that is generated from the sourcecollector module SO. In the meantime, the gas 271 may be continuouslyprovided to the source vessel 110 after the halting. In someembodiments, the master controller 157 receives an output signal fromthe energy sensor 170 (shown in FIG. 2A) and performs an analysis basedat least in part on this received output to actuate the control unit155. In some embodiments, the control unit 155 further actuates thescanner 200 to calibrate a next processed substrate W2.

Returning to FIG. 1, the method M then proceeds to block S105 where thefirst gas exhaust rate of the vessel is increased to a second gasexhaust rate by increasing an open ratio of a valve in a pump, in whichthe valve is connected to the vessel. With reference to FIG. 3, in someembodiments of block S105, the control unit 155 therefore provides asignal to the converter 158 to increase the open ratio of the sourcevalve 208. That is, the controller 155 configured to increase open ratioof the source valve 208 in response to a turn-off operation of the fueldroplet generator 120 and a turn-off operation of the laser source 102.In some embodiments, the open ratio of the source valve 208 shown inFIG. 3 is to increase in a range from about 30% to about 50% and thereis no plasma 114 as shown in FIG. 2B generated during the adjusting ofthe open ratio of the source valve 208. Thus, control of the pump 224 inresponse to the adjustment of the source valve 208 may be used to higherthe second gas exhaust rate than about 400 Ls. In some embodiments,pressure in the vessel is lowered to a second pressure less than thefirst pressure mentioned-above in response to the change of the gasexhaust rate. In some embodiments, the pressure may be lower to be lessthan about 1 mm Bar in the source vessel 110 in response to the changeof the gas exhaust rate. Hence, a flow rate of the gas through thesource vessel 110 may be enhanced so as to increase the efficiency ofcarrying the debris away from the source vessel 110. In someembodiments, the gas exhaust rate of the source vessel 110 shown in FIG.3 that in response to the adjustment of the source valve 208 isdifferent from the gas exhaust rate of the source vessel 110 shown inFIG. 2B. In some embodiments, a pressure in the source vessel 110 shownin FIG. 3 that in response to the adjustment of the valve 208 isdifferent from a pressure in the source vessel 110 shown in FIG. 2B.

In some embodiments, the second gas exhaust rate is greater than about350 L/s to increase the efficiency of carrying the debris away from thesource vessel 110 shown in FIG. 3. In some embodiments, the second gasexhaust rate is greater than about 450 Ls to increase the efficiency ofcarrying the debris away from the source vessel 110 shown in FIG. 3.

In some embodiments, the open ratio of the source valve 208 ismaintained during the alignment process of the substrate W2 and duringthe providing of the gas 271 to the source vessel 110. Further, the gasexhaust rate of the source vessel 110 is maintained, and thus thepressure in the source vessel is substantially a constant.

Returning to FIG. 1, the method M then proceeds to block S106 where asecond substrate is calibrated, that the second substrate will beexposed to a second EUV radiation directing from the vessel, duringproviding of the gas in the vessel at the second gas exhaust rate. Withreference to FIG. 3, in some embodiments of block S106, the nextprocessed substrate W2, that will be exposed to an EUV radiationdirecting from the vessel, is provided.

FIG. 3 shows a schematic example of a field image alignment arrangementin the scanner 200. Such an alignment arrangement is based on a staticmeasurement. The field image alignment arrangement of FIG. 3 comprises alight source 301, which is a broadband source. The light source 301 isconnected to one end of a fiber 302. A transmitter 303 is connected tothe opposite end of the fiber 302. Optics to provide an alignment beamtowards a mark on a substrate W2 include a semi-transparent mirror 304and a mirror 305. Imaging optics 306 are provided to receive alignmentradiation back from the mark M3 and to provide a suitable optical imageto a detector 307, e.g., a charged coupled device (CCD). The detector307 is connected to a processor 308. The processor 308 in its turn isconnected to an actuator 311 and a memory 156. The actuator 311 isconnected to the substrate table WT, on which the substrate W2 can beplaced.

In use, the light source 301 produces a broadband light beam that isoutput via the fiber 302 to the transmitter 303. The transmitter 303provides a broadband light beam 309 that is reflected by thesemi-transparent mirror 304 to the mirror 305. Mirror 305 produces abroadband light beam 310 to be directed to the mark on the substrate W2.The broadband light beam 310 impinging on the mark is reflected back asalignment radiation to the mirror 305. The mirror 305 reflects thereceived light to the semi-transparent mirror 304 which passes at leasta portion of the received light to the imaging optics 306. The imagingoptics 306 is arranged to collect the received alignment radiation andto provide a suitable optical image to the detector 307. The detector307 provides an output signal to the control unit 155 that depends onthe content of the optical image received from the imaging optics 306.The output signal that is received from the detector 307 as well asresults of actions performed by the control unit 155 may be stored in amemory. The control unit 155 calculates a position of the alignment markbased on one or more of the output signal it receives from the detector307. It then provides a further output signal to the actuator 311. Theactuator 311 is arranged to move substrate table WT. Upon reception ofthe further output signal the actuator 11 moves the substrate table WTtowards a desired position.

Returning to FIG. 1, the method M then proceeds to block S107 where thegas is directed from a collector toward an intermediate focus (IF) unitin the vessel at the second gas exhaust rate, so as to carry the debrisaway from the collector to remove the debris from the vessel. Withreference to FIG. 3, in some embodiments of block S107, during theproviding of the gas in the source vessel 110 at the higher gas exhaustrate, the gas is directed from a collector mirror 118 toward anintermediate focus (IF) unit 130 in the source vessel 110 and furthertoward the pump 224, in response to the adjusting of the open ratio ofthe source valve 208. Thus, the debris is carried away from thecollector mirror 118 by the gas, passes through a plasma site where theplasma 114 is generated as shown in FIG. 2B, and is further removed fromthe source vessel 110.

In some embodiments, the EUV lithography system 10 may include filter159. The filter 159 may function to remove at least a portion of atarget species, e.g. contaminants that may degrade optical componentsand/or absorb EUV light, from gas flowing in the flow path. For example,when a tin containing material is used as a source material to generatethe plasma, contaminants such as tin hydrides, tin oxides and tinbromides may be present in the gas which may degrade optical componentsand/or absorb EUV light. These contaminants may be removed using one ormore suitable filters, e.g. zeolite filters, cold traps, chemicalabsorbers, etc.

Returning to FIG. 1, the method M then proceeds to block S108 where anEUV photomask is calibrated. The EUV photomask will be exposed to thesecond EUV radiation directing from the vessel, during providing of thegas in the vessel at the second gas exhaust rate. With reference to FIG.4, in some embodiments of block S108, the scanner 200 of the EUVlithography system 10 further includes a first positioner PM and asecond positioner PW. The first positioner PM is connected to thesupport structure (e.g. a mask table) MT and is configured to accuratelyposition the patterning device. The second positioner PW is connected tothe substrate table (e.g. a wafer table) WT and is configured toaccurately position the substrate W2. The projection system PS isconfigured to project a pattern imparted to the patterned beam 260 bythe patterning device MA onto a target portion C (e.g. comprising one ormore dies) of the substrate W2.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

Referring to FIG. 4, the illumination system IL receives an extremeultra violet (EUV) radiation beam from the source vessel 110. Theradiation beam 210 from the illumination system IL is incident on thepatterning device (e.g. mask) MA, which is held on the support structure(e.g. mask table) MT, and is patterned by the patterning device. Afterbeing reflected from the patterning device (e.g. mask) MA, the patternedbeam 260 passes through the projection system PS, which focuses the beamonto a target portion C of the substrate W2. With the aid of the secondpositioner PW and position sensor PS2 (e.g. an interferometric device,linear encoder or capacitive sensor), the substrate table WT can bemoved accurately, e.g. so as to position different target portions C inthe path of the radiation beam 210 and the patterned beam 260.Similarly, the first positioner PM and another position sensor PS1 canbe used to accurately position the patterning device (e.g. mask) MA withrespect to the path of the radiation beam B. Patterning device (e.g.mask) MA and substrate W2 may be aligned using mask alignment marks M1and M2 and substrate alignment marks P1 and P2.

The depicted apparatus could be used in at least one of the followingmodes:

Firstly, in step mode, the support structure (e.g. mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

Secondly, in scan mode, the support structure (e.g. mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e. a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the support structure (e.g. mask table)MT may be determined by the (de-) magnification and image reversalcharacteristics of the projection system PS.

Thirdly, in another mode, the support structure (e.g. mask table) MT iskept essentially stationary holding a programmable patterning device,and the substrate table WT is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Referring to FIG. 4, the control unit 155 therefore provides a signal tothe converter 158 to adjust the open ratio of the source valve 208. Insome embodiments, the open ratio of the source valve 208 shown in FIG. 4is in a range from about 30% to about 50% and there is no plasma 114 asshown in FIG. 2B generated during the adjusting of the open ratio of thesource valve 208. Thus, control of the pump 224 in response to theadjustment of the source valve 208 may be used to control the gasexhaust rate in the source vessel 110 to be greater than about 400 L/s.In some embodiments, the pressure may be less than about 1 mm Bar in thesource vessel 110 in response to the change of the gas exhaust rate.Hence, a flow rate of the gas through the source vessel 110 may beenhanced so as to increase the efficiency of carrying the debris awayfrom the source vessel 110. In some embodiments, the gas exhaust rate ofthe source vessel 110 shown in FIG. 4 that in response to the adjustmentof the source valve 208 is different from the gas exhaust rate of thesource vessel 110 shown in FIG. 2B. In some embodiments, a pressure inthe source vessel 110 shown in FIG. 4 that in response to the adjustmentof the source valve 208 is different from a pressure in the sourcevessel 110 shown in FIG. 2B.

In some embodiments, the gas exhaust rate is greater than about 350 L/sto increase the efficiency of carrying the debris away from the sourcevessel 110 shown in FIG. 4. In some embodiments, the gas exhaust rate isgreater than about 450 Ls to increase the efficiency of carrying thedebris away from the source vessel 110 shown in FIG. 4.

In some embodiments, the open ratio of the source valve 208 ismaintained during the alignment process of the photomask and during theproviding of the gas 271 to the source vessel 110. Further, the gasexhaust rate of the source vessel 110 is maintained, and thus thepressure in the source vessel is substantially a constant.

During the providing of the gas in the source vessel 110 at the highergas exhaust rate, the gas is directed from a collector mirror 118 towardan intermediate focus (IF) unit 130 in the source vessel 110 and furthertoward the pump 224, in response to the adjusting of the open ratio ofthe source valve 208. Thus, the debris is carried away from thecollector mirror 118 by the gas, passes through a plasma site where theplasma 114 is generated as shown in FIG. 2B, and is further removed fromthe source vessel 110.

Returning to FIG. 1, the method M then proceeds to block S109 where thesecond exhaust rate of the vessel is decreased to the first exhaust rateby decreasing the open ratio of the valve for the next processing in thevessel, such as a calibrating process on the intensity of a second EUVradiation. With reference to FIG. 5, in some embodiments of block S109,the master controller 157 is determined that the laser pulses properlyintercept the droplets in the right place and time for effective andefficient EUV light production for a calibrating process on theintensity of EUV radiation which will be carried out later. When theamplified light beam 110 strikes the fuel droplets 112, the fueldroplets 112 is converted into a plasma state that has an element withan emission line in the EUV range.

At the same time, the master controller 157 receives an output from theenergy sensors 170 (shown in FIG. 2A) and performs an analysis based atleast in part on this received output to actuate the control unit 155.The control unit 155 therefore provides a signal to the converter 158 todecrease the open ratio of the source valve 208. In some embodiments,the open ratio of the source valve 208 shown in FIG. 5 is decreased in arange from about 20% to about 30%. Thus, control of the pump 224 inresponse to the adjustment of the source valve 208 may be used todecrease the gas exhaust rate to be less about 400 L/s. In someembodiments, pressure in the vessel is increased to be higher than thesecond pressure mentioned-above in response to the change of the gasexhaust rate. In some embodiments, control of the pump 224 in responseto the adjustment of the source valve 208 may be used to maintain aselected gas number density in the irradiation region 122 of the sourcevessel 110 and/or pressure gradient and/or to maintain a selected flowrate through the source vessel 110 and or to maintain a selected gascomposition, e.g. a selected ratio of several gases, e.g. H₂, HBr, He,etc. Therefore, amount of the plasma 114 can be generated for creatingeffective and efficient EUV light.

In some embodiments, the gas exhaust rate is less than about 350 L/s tomaintain a selected gas number density and/or a selected gas compositionin the irradiation region 122 of the source vessel 110 shown in FIG. 5.In some embodiments, the gas exhaust rate is less than about 450 Ls tomaintain a selected gas number density and/or a selected gas compositionin the irradiation region 122 of the source vessel 110 shown in FIG. 5.

In some embodiments, the open ratio of the source valve 208 ismaintained during a next processed calibrating process on an intensityof the EUV radiation for the next processed substrate and during theproviding of the gas 271 to the source vessel 110. Further, the gasexhaust rate of the source vessel 110 is maintained, and thus thepressure in the source vessel is substantially a constant.

Returning to FIG. 1, the method M then proceeds to block SI where anintensity of the second EUV radiation generated in the vessel iscalibrated. In some embodiments, a photolithography process will beperformed on the substrate W2 after the calibration of the intensity ofthe EUV radiation. With reference to FIG. 5, in some embodiments ofblock S110, the EUV energy monitor 150 is configured to sampledifferences in energy up and down and left and right around the lightbeam 104 to determine the positional relationship between the light beam104 and the irradiation region 122. The master controller 157 receivesan output from the EUV energy monitor 150 and performs an analysis basedat least in part on this received output to determine the relativealignment between the drive axis of the amplified light beam 104 andplasma 114.

The EUV lithography system 10 can also include a guide laser 175 thatcan be used to align various sections of the EUV lithography system 10or to assist in steering the amplified light beam 104 to the irradiationregion 122. In connection with the guide laser 175, the EUV lithographysystem 10 includes a sampling apparatus 124 (see FIG. 2A) that is placedwithin the focusing system 108 to sample a portion of light from theguide laser 175 and the amplified light beam 104. In otherimplementations, the sampling apparatus 124 is placed within the beamtransport system 106. The sampling apparatus 124 can include an opticalelement that samples or re-directs a subset of the light, such opticalelement being made out of any material that can withstand the powers ofthe guide laser beam and the amplified light beam 104. The samplingapparatus 124 can include an optical sensor that captures images ofdiagnostic portions of the sampled light, and the optical sensor canoutput an image signal that can be used by the master controller 157 fordiagnostic purposes. In FIG. 5, the master controller 157 analyzes theoutput from the EUV energy monitor 150 and uses this information toadjust components including the focusing system 108, the beam transportsystem 106, guide laser 175, and/or sampling apparatus 124 shown in FIG.2A.

In some embodiments, the alignment process of the substrate W2, thealignment process of the photomask, the calibrating process on theintensity of the EUV radiation, and the photolithography process aredescribed with reference to FIG. 6. It should be understood that thephotolithography process and calibrating process on the intensity of theEUV radiation the can be implemented in a wafer exposure process P1, andthe alignment processes of the substrate and the photomask can beimplemented in a lot overhead process P2 as used throughout this text.In some embodiments, an open ratio R2 of the source valve 208 used inthe lot overhead process P2 is greater than an open ratio R1 of thesource valve 208 used in the wafer exposure process P1, and thus the gasexhaust rate of the source vessel 110 in response to the open ratio R2of the source valve 208 is less than that in response to the open ratioR1 of the source valve 208. In some embodiments, the open ratios R1 andR2 are substantially constant. In some embodiments, the wafer exposureprocess P1 is performed at a duration time T1 greater than a durationtime T2 performed by the lot overhead process P2.

Based on the above discussion, it can be seen that the presentdisclosure offers advantages. It is understood, however, that otherembodiments may offer additional advantages, and not all advantages arenecessarily disclosed herein, and that no particular advantages isrequired for all embodiments.

One advantage is that the open ratio of the valve used in the lotoverhead process exhibits strong ability to exclude tin dusts from thesource vessel compared to that used in the wafer exposure process, andthus improve vessel cleanness to extending lifetime of the sourcecollector.

Another advantage is that the valve with greater open ratio compared tothat used in the wafer exposure process may function to remove thetarget species, e.g. contaminants that may degrade optical componentsand/or absorb EUV light, from gas flowing in the flow path, and thus theEUV light may be generated effective and efficient.

Another advantage is that the open ratio of the valve used in the waferexposure process is different from that used in the lot overheadprocess, so as to maintain a selected gas number density in theirradiation region of the source vessel during the exposure process forcreating effective and efficient EUV light.

In some embodiments, a method includes generating a plasma that emits afirst EUV radiation in a vessel at a first gas exhaust rate of thevessel; directing the first EUV radiation to a first substrate using acollector in the vessel; halting the generating of the first EUVradiation; and ejecting a gas past the collector at a second gas exhaustrate of the vessel, in which the second gas exhaust rate is greater thanthe first gas exhaust rate after the halting.

In some embodiments, a method includes providing a gas in a vessel;generating a plasma that emits an EUV radiation in the vessel duringproviding the gas; halting the generating of the EUV radiation and keepproviding the gas; and increasing an open ratio of a source valve thatis connected to a pump, wherein the source valve is in gas communicationwith the vessel.

In some embodiments, a system includes an EUV source vessel, a fueldroplet generator, a laser source, a collector, a gas source, a pump, agas line, a source valve, and a controller. The fuel droplet generatoris connected to the EUV source vessel. The laser source is connected tothe EUV source vessel. The collector is disposed in the EUV sourcevessel and has an opening. The gas source is configured to provide a gasthrough the opening of the collector. The gas line connects the pump andthe EUV source vessel. The source valve is in the gas line. Thecontroller is configured to increase an open ratio of the source valvein response to a turn-off operation of the fuel droplet generator.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method, comprising: generating a plasma that emits a first EUVradiation in a vessel while the vessel is exhausted at a first gasexhaust rate; directing the first EUV radiation to a first substrateusing a collector in the vessel, wherein the first EUV radiation passesthrough a vane structure that laterally surrounds the collector and isconnected to an upper end surface of the collector, the vane structurecomprises an inner sidewall, an outer sidewall laterally surrounding theinner sidewall and shorter than the inner sidewall, and a topmostsurface sloped along a direction to form an acute angle with a centeraxis of the collector, and an interior of the acute angle that extendsaway from a vertex of the acute angle formed by the center axis of thecollector and the topmost surface overlaps the collector; monitoring anintensity of the first EUV radiation; and exhausting the vessel at asecond gas exhaust rate in response to the monitored intensity of thefirst EUV radiation, wherein the second gas exhaust rate is greater thanthe first gas exhaust rate.
 2. The method of claim 1, further comprisingcalibrating a second substrate during exhausting the vessel at thesecond gas exhaust rate, wherein the second substrate is exposed to asecond EUV radiation directed through the collector in the vessel afterexhausting the vessel at the second gas exhaust rate is complete.
 3. Themethod of claim 1, further comprising calibrating an EUV photomaskduring exhausting the vessel at the second gas exhaust rate, wherein theEUV photomask is exposed to a second EUV radiation directed through thecollector in the vessel after exhausting the vessel at the second gasexhaust rate is complete.
 4. The method of claim 1, further comprisingcalibrating an intensity of a second EUV radiation that generates in thevessel after exhausting the vessel at the second gas exhaust rate iscomplete.
 5. (canceled)
 6. The method of claim 1, wherein the first gasexhaust rate is performed at a first duration time and the second gasexhaust rate is performed at a second duration time that is less thanthe first duration time.
 7. The method of claim 1, wherein the first gasexhaust rate is substantially constant.
 8. The method of claim 1,wherein the second gas exhaust rate is substantially constant.
 9. Themethod of claim 1, wherein the generating of the plasma is performedsuch that debris falls on the collector, and the exhausting the vesselat the second gas exhaust rate is performed such that the debris iscarried away from the collector.
 10. The method of claim 9, wherein thedebris comprises tin.
 11. (canceled)
 12. A method, comprising:providing, in a vessel, a gas passing through a collector and a vanestructure laterally surrounding the collector and connected to an upperend surface of the collector, wherein the vane structure has an innersidewall and an outer sidewall laterally surrounding the inner sidewall,and a topmost position of the outer sidewall is lower than a topmostposition of the inner sidewall; generating a plasma that emits an EUVradiation in the vessel during providing the gas; halting the generatingof the EUV radiation and while continuing to provide the gas; andincreasing an open ratio of a source valve that is connected to a pump,wherein the source valve is in gas communication with the vessel, and animaginary plane formed by extending along and past a top surface of thevane structure passes through an outlet of the vessel that is in gascommunication with the source valve.
 13. The method of claim 12, whereinthe increasing the open ratio of the source valve is performed after thehalting of the generating of the EUV radiation.
 14. The method of claim13, wherein the increasing the open ratio of the source valve isperformed such that a first gas exhaust rate of the vessel after thehalting is greater than a second gas exhaust rate of the vessel beforethe halting.
 15. The method of claim 14, wherein the first gas exhaustrate is performed at a first duration time and the second gas exhaustrate is performed at a second duration time that is less than the firstduration time.
 16. The method of claim 12, wherein the providing of thegas is substantially performed at a constant flow rate.
 17. The methodof claim 12, wherein the generating of the plasma is performed such thatdebris falls on the collector, and the increasing of the open ratio ofthe source valve is performed such that the gas carry the debris awayfrom the collector. 18-19. (canceled)
 20. A system, comprising: an EUVsource vessel; a fuel droplet generator connected to the EUV sourcevessel; a laser source connected to the EUV source vessel; a collectorin the EUV source vessel and having an opening; a gas source configuredto provide a gas through the opening of the collector; a vane structureabove and laterally surrounding the collector and connected to an upperend surface of the collector, wherein the vane structure comprises aninner sidewall and an outer sidewall laterally surrounding the innersidewall and shorter than the inner sidewall, and the inner sidewall ofthe vane structure is coplanar with an inner sidewall of the EUV sourcevessel immediately adjacent to an exit aperture of the EUV sourcevessel; a pump; a gas line connecting the pump and the EUV sourcevessel; a source valve in the gas line; and a controller configured toincrease an open ratio of the source valve in response to a turn-offoperation of the fuel droplet generator.
 21. The system of claim 20,wherein an imaginary plane formed by extending along and past a topsurface of the vane structure passes through an outlet of the EUV sourcevessel that is connected to the gas line.
 22. (canceled)
 23. The methodof claim 1, wherein the first and second gas exhaust rates are measuredby a gas monitor that is surrounded by the vane structure and above theupper end surface of the collector.
 24. The method of claim 12, whereinthe increasing the open ratio of the source valve is performed such thata first gas exhaust rate of the vessel after the halting is differentthan a second gas exhaust rate of the vessel before the halting, andwherein the first and second gas exhaust rates are measured by a gasmonitor that is disposed between the outer sidewall of the vanestructure and an inner sidewall of the vessel.
 25. The system of claim20, wherein the vane structure comprises a topmost surface sloped alonga direction to form an acute angle with a center axis of the collector,and an interior of the acute angle that extends away from a vertex ofthe acute angle formed by the center axis of the collector and thetopmost surface overlaps the collector.